1. Field of the Invention
This invention relates to copolymers of C.sub.1 -C.sub.5 alkyl vinyl ether and maleic acid, and, more particularly, to colorless, stabilized aqueous solutions of such copolymers.
2. Description of the Prior Art
The copolymerization of maleic anhydride and a C.sub.1 -C.sub.5 alkyl vinyl ether yields a copolymer which is more properly characterized as an interpolymer wherein both ethylenically unsaturated moieties enter into the formation of the same polymer chain. It has been indicated by infra-red spectroscopy, nuclear magnetic resonance spectroscopy and other chemical analyses that alkyl vinyl ethers and maleic anhydride interpolymerize in the molar ratio of about 1:1, the polymer chain thereof containing alternating anhydride units and alkyl vinyl ether units. The resulting interpolymers are, in general, soluble in aqueous systems upon prolonged contact with water, due to the hydrolysis of the anhydride groups to acid groups, and are known to be useful in personal care products and other applications. In the aqueous environment, however, it has been found that the interpolymers undergo substantial and serious decomposition, which is manifested by a decrease in the viscosity of solutions prepared from the interpolymer materials. As it is often necessary to prepare specific interpolymers of specified viscosity grade, it is often found that, after suitable processing of the interpolymer the expected viscosity specifications of the solutions are not met. It has further been ascertained that elevated temperatures and even storage under ambient conditions cause rapid decline in the viscosity grade of the material. Since it is essential in commercial practice to obtain such interpolymers at definite specifications of viscosity, it has become necessary to effect a stabilization of the interpolymers whereby the viscosity of the material will remain substantially unchanged under conditions which normally lead to decomposition and consequent lowering of the viscosity.
In the art, various procedures have been attempted in order to obtain aqueous solutions of the interpolymers which will retain constant viscosities during extended periods of storage. These prior art methods, however, have not been entirely successful as in each instance the viscosity has been found to decrease over substantial periods of storage. These methods have generally involved the addition to the aqueous solutions of the interpolymer of various materials in an effort to effect the desired stabilization of the interpolymers. It has been suggested, for example, that certain thioamides may be employed to stabilize aqueous solutions of the interpolymer. However, these materials have been found useful only for short periods as the stabilizing characteristics become very poor over extended periods. Similarly, the use of ethylenediaminetetraacetic acid (EDTA) has been suggested as a stabilizing agent. As with the thioamides, however, this material has not accomplished the purpose.
Therefore, as pointed out, none of these prior materials have proven successful in providing aqueous solutions of interpolymers or copolymers of maleic acid or anhydride with alkyl vinyl ethers to enable use of the solutions for commercial specifications over periods of extended storage.
Kervenski, in U.S. Pat. No. 3,531,427, for example, described a stabilizer mixture of ethylenediaminetetraacetic acid and a secondary additive selected from the group consisting of thiourea, allylthiourea, ammonium thiocyanate, tannic acid, pyrrolidone, vinyl pyrrolidone, pyrogallol, dimethyl sulfoxide, polyvinyl pyrrolidone, sodium diethyldithiocarbamate, resorcinol, gallic acid, 2,2',4,4'-tetrahydroxybenzophenone, phloroglucinol, tartaric acid, citric acid, gluconic acid, and glycolic acid.
However, it has been discovered, that, for one or more of the following reasons, the stabilizer mixtures disclosed by Kervenski were not entirely satisfactory:
(1) many of the secondary additives listed were not effective in stabilizing the viscosity of the interpolymer;
(2) those secondary additives which were effective had to be used in very large amounts which precluded the use of the interpolymer in personal care and industrial applications;
(3) most of the secondary additives were very toxic even in low concentrations;
(4) several of the secondary additives were not sufficiently soluble in water to provide stabilized aqueous solutions of the copolymer; and
(5) the secondary additives discolored the solution.
Accordingly, it is an object of this invention to provide colorless, stabilized aqueous solutions of a copolymer of a C.sub.1 -C.sub.5 alkyl vinyl ether and maleic acid which is suitable for personal care and industrial applications, and which is effective for stabilizing the copolymer even when used at low concentrations.
Another object of the invention is to provide a process of preparing such colorless, stabilized aqueous solutions having higher viscosities than available previously and which will retain their initial viscosity levels over extended periods of time.
These and other objects and features of the invention will be made apparent as the description thereof proceeds.